An addition-curable silicone composition includes an organopolysiloxane containing an aliphatic unsaturated group such as an alkenyl group to provide a cured product by being cured by a hydrosilylation reaction. A cured product to be thus obtained is excellent in heat resistance, cold resistance and electric insulation, and due to its transparent appearance, it is used for various optical purposes.
Since a silicone cured product used for optical purposes requires higher transparency and refractive index, a method for using a linear organopolysiloxane having an aryl group such as a phenyl group in a main skeleton is conventionally used to satisfy these requirements (Patent Literatures 1 and 2).
Toward achieving higher hardness, a composition including a branched organopolysiloxane having a unit represented by RSiO3/2, wherein R represents an organic group is used (Patent Literatures 3 to 5).
However, the above types of curable organopolysiloxane compositions are prone to such problems as insufficient temperature cycling resistance, peeling from a substrate and crack generation of a cured product obtained by curing each composition.